1. Field of the Invention
This invention relates to an image display apparatus that utilizes a hologram element.
2. Description of the Related Art
An image display apparatus, that irradiates image light from various liquid projectors to a hologram screen formed by bonding a hologram element to light-transmissible films, reproduces images on the screen and offers the images to observers, is known.
Each light-transmissible film described above is pulled in a longitudinal direction V and in a transverse direction H during its production process as shown in FIG. 3 that will be described later in more detail. A symbol RW represents a roll winding direction. This pulling operation orients the molecules of the light-transmissible film and the film becomes an anisotropic material. Therefore, birefringence occurs when rays of light are incident into the light-transmissible film from directions other than a specific direction.
When the rays of light are incident into a material that exhibits birefringence, the rays are divided into normal rays and abnormal rays depending on the direction of a vibration surface of the light, and travel inside the material.
Therefore, when image light 91 is irradiated to the hologram screen 2 as shown in FIG. 12, this image light 91 is divided into two rays of light 911 and 912 inside the light-transmissible film 21 that is the anisotropic material, and these rays enter, under such a state as described above, into the hologram element 20. In consequence, these two rays of light 911 and 912 interfere with each other inside the hologram element 20.
Due to this interference, an interference pattern 99 is displayed on the hologram screen 2 in superposition with the image 12 reproduced from the image light 91, so that image quality of the image 12 is greatly deteriorated.
This problem manifests itself particularly in a large-scale hologram screen.
When the rays of light containing a mixture of S polarized light and P polarized light enter into a material that exhibits birefringence, the S polarized light component and the P polarized light component interfere with each other because the refractive index in the material varies depending on the direction of polarization.
Therefore, there develops a problem that, if the rays of light containing the mixture of S polarized light and P polarized light are incident when the image is displayed on the hologram screen 2, the interference pattern appears in superposition with the image 12 on the hologram screen 2. Incidentally, most of recent liquid crystal projectors are of a type that projects a mixture of S polarized light and P polarized light.
In view of the problems of the prior art described above, the present invention aims at providing an image display apparatus capable of offering high-quality images without superposition of an interference pattern with the images on a hologram screen.
According to the first aspect of the present invention, there is provided an image display apparatus including a hologram screen formed by bonding a hologram element to light-transmissible films and an illumination device for irradiating image light to the hologram screen and reproducing an image on the hologram screen, wherein: the light-transmissible film positioned on the irradiation side of the image light relative to the hologram element has an angle of deviation of not greater than 45 degrees between a direction of the light-transmissible film that gives the highest thermal shrinkage ratio and an axial direction in the hologram screen.
Here, the term xe2x80x9caxial directionxe2x80x9d is a direction parallel to a segment D obtained by rotating a segment B by 90xc2x0 along an arc C with G1 as the center when a segment connecting the center G1 of the hologram screen and an irradiation center G2 of the irradiation device is referred to as a segment A, a perpendicular at the center G1 of the hologram screen is referred to as a perpendicular B, and an arc extending from the segment A to the segment B with a fan center at G1 is referred to as the arc C.
It is most noteworthy, in the present invention, that the angle of deviation between a direction of the light-transmissible film on the irradiation side of the image light to the hologram device, which direction gives the highest thermal shrinkage ratio, and the axial direction in the hologram screen is within 45 degrees.
Incidentally, the term xe2x80x9cwithin 45 degreesxe2x80x9d represents the angle in either clockwise or counter-clockwise directions.
Next, the operation of the first embodiment of the present invention will be explained.
The light-transmissible film is pulled in both longitudinal and transverse directions during its production process as shown in FIG. 3. This pulling operation orients the molecules of the light-transmissible film, and the film becomes an anisotropic material. The orientation condition of the molecules in the light-transmissible film varies depending on the thickness of the light-transmissible film, its production method, a manufacturing machine, and so forth, but generally has the tendency shown in FIG. 4.
The thermal shrinkage ratio of the light-transmissible film made of the anisotropic material varies depending on direction. The thermal shrinkage ratio is large in the orientation direction of the molecules and is small in directions different from the molecular orientation direction.
When the orientation direction of the molecules is coincident with the axial direction of the hologram screen, the optical axis of the incident image light orthogonally crosses the plane on which a slice plane of a refractive index ellipse of the light-transmissible film describes a circle. Therefore, birefringence does not occur in the same way as in isotropic materials.
The first embodiment stipulates that the angle between the direction of the light-transmissible film giving the highest thermal shrinkage ratio and the axial direction of the hologram screen is not greater than 45 degrees. Therefore, the angle between the orientation direction of the molecules and the axial direction of the hologram screen is also within 45 degrees.
In the hologram screen according to the present invention, therefore, birefringence of the light-transmissible film does not easily occur so that almost no interference pattern occurs. Even when the interference pattern occurs, it is not very noticeable.
In this way, image quality is prevented from being deteriorated by the interference pattern.
When the angle between them is greater than 45xc2x0, a noticeable interference pattern is displayed in superposition with the image, and image quality of the image is greatly spoiled.
As described above, the first embodiment can provide an image display apparatus capable of offering high-quality images without superposition of an interference pattern with the image on a hologram screen.
The hologram screen according to the first embodiment can provide the effect described above when the direction of the light-transmissible film on the incidence side of the image light, that gives the highest thermal shrinkage ratio, is aligned with the axial direction of the hologram screen.
When the light-transmissible films are so disposed as to sandwich the hologram element, the direction of both light-transmissible films that gives the highest thermal shrinkage ratio can be aligned with the axial direction of the hologram screen.
In the hologram screen, it is possible to employ a construction in which the light-transmissible film having the direction giving the highest thermal shrinkage ratio and aligned with the axial direction of the hologram screen is disposed on the incidence side of the image light, and a transparent substrate made of a transparent isotropic material such as glass is disposed on the opposite side.
The light-transmissible film may be a colorless transparent film, or a thinly colored transparent film.
Further, a plurality of light-transmissible films may be bonded to the hologram element.
Various plastic films can be used for the light-transmissible film. Besides a PET film, to be later described, it is possible to use TAC (triacetylcellulose), polycarbonate, polyurethane, polystyrene, polyethylene, nylon, vinyl chloride, polypropylene and polyvinyl alcohol.
The hologram element used in the present invention can be produced by the steps of using diffused light through a light diffusion member such as frosted glass as object light and non-diffused light as reference light, projecting both of them to a photosensitive member, and recording an interference fringe functioning as a diffraction grating formed by them on the photosensitive member.
Various devices capable of projecting projection light of still images and dynamic images such as a slide projector, OHP (overhead projector), a projector, a movie projector, and so forth, can be used as the illumination device in the first embodiment.
The illumination device described above has an image supply device inside or outside thereof, and functions as a source of the image light. A reproducing device such as video tapes, various optical disks, etc, and personal computers can be used as the image supply device, and the source of the image light can be supplied from outside through a communication line.
Incidentally, when a rectangular hologram screen is used in the image display apparatus according to the first embodiment, the apparatus is installed in most cases in parallel with the floor or the ground, for example, but may also be disposed slantingly.
When the hologram screen is rectangular or square and the bottom side of the screen is disposed in parallel with the ground surface or the floor, the axial direction described above coincides with the perpendicular direction.
The hologram screen can be installed on a curved glass window, or the like. Beside the wall surface, the hologram screen may also be installed on the ceiling surface or the floor surface.
The hologram screen may have various shapes such as a circle, a trapezoid and a triangle.
According to the second aspect of the present invention, angle deviation between the direction of the light-transmissible film giving the highest thermal shrinkage ratio and the axial direction in the hologram screen is preferably within 15 degrees. When this angle deviation is within 45 degrees, the interference pattern is hardly noticeable as already described. When the angle deviation is within 15 degrees, the occurrence itself of the interference pattern can almost be suppressed.
According to the third aspect of the present invention, the direction of the light-transmissible film giving the highest thermal shrinkage ratio and the axial direction of the hologram screen preferably coincide with each other.
According to this construction, an excellent image display apparatus almost free of the occurrence of an interference pattern can be obtained.
The direction giving the highest thermal shrinkage ratio can be determined by, for example, cutting out, at 5 degree intervals, strips from a hologram screen and comparing the thermal shrinkage ratios of the screen fragments.
Next, according to the fourth aspect of the present invention, there is provided an image display apparatus including a hologram screen formed by bonding a hologram element to light-transmissible films and an illumination device for irradiating image light to the hologram screen and reproducing an image on the hologram screen, wherein the light-transmissible film positioned on the irradiation side of the image light relative to the hologram element is so constituted as not to generate birefringence.
The light-transmissible film according to the fourth aspect has an isotropic refractive index, and does not generate birefringence from which direction the rays of light may be incident thereto. Therefore, it becomes possible to acquire an excellent image display apparatus substantially free from the interference pattern irrespective of the bonding angle of the films.
In the fourth aspect described above, the term xe2x80x9clight-transmissible film not generating birefringencexe2x80x9d means a film in which birefringence does not occur in at least 60% of the screen area.
Even though birefringence does not occur in at least 60% of the area, the interference pattern occurs in remaining less than 40% of the area. However, the quantity of the occurrence of the interference pattern is small, and the image having an image quality sufficient enough to satisfy the requirements of viewers can be obtained.
When the portions in which the interference pattern occurs are less than 40%, an evaluation result can be obtained, in a sensory test, to the effect that the interference pattern portions are not distracting.
Incidentally, the term xe2x80x9clight-transmissible film free from occurrence of birefringencexe2x80x9d generally represents those films which are commercially available as xe2x80x9cfilms having a high optical gradexe2x80x9d. Such films are produced so that each part of the film has a uniform refractive index so as not to generate birefringence.
According to the fifth aspect of the present invention, the light-transmissible film preferably comprises a resin film. Since many transparent resin films having high flexibility are available, they are suitable for bonding to the hologram element.
Besides the polyethylene terephthalate film, to be later described in more detail, the resin film can use TAC (triacetylcellulose), polycarbonate, polyurethane, polystyrene, polyethylene, nylon, vinyl chloride, polypropylene, polyvinyl alcohol, or the like.
According to the sixth aspect of the present invention, there is provided an image display apparatus including a hologram screen formed by bonding a hologram element to light-transmissible films and an illumination device for irradiating image light to the hologram screen and reproducing an image on the hologram screen, wherein the light-transmissible film positioned on the irradiation side of the image light relative to the hologram element has a difference of not greater than 0.3% between a direction giving the highest thermal shrinkage ratio and a direction giving the lowest thermal shrinkage ratio.
The light-transmissible film according to the sixth aspect does not have a difference in the thermal shrinkage ratio as a whole and optically functions as an isotropic material as a whole. Therefore, it becomes possible to obtain an image display apparatus free from interference fringes, etc, due to birefringence, and capable of providing high-quality images.
When the difference of the thermal shrinkage ratio is greater than 0.3%, the interference fringe is likely to occur, thereby lowering image quality.
Incidentally, to measure the thermal shrinkage ratio, fragments are cut out from the light-transmissible film at different angles, and the thermal shrinkage ratio of each fragment is examined to determine the thermal shrinkage ratio.
According to the seventh aspect of the present invention, the light-transmissible film positioned on the irradiation side of the image light relative to the hologram element preferably comprises a polyethylene terephthalate film.
The polyethylene terephthalate (hereinafter called xe2x80x9cPETxe2x80x9d) film is excellent in weatherability and transparency, is tough and is therefore suitable for protecting the hologram element.
According to the eighth aspect of the present invention, it is preferred to bond a plurality of hologram elements through the light-transmissible films.
Since the image from the irradiated image light is displayed in each of these hologram elements, the image finally entering the eyes of the observer is the image superposed in the number of the hologram elements.
Therefore, an image that has high brightness and is easy to watch can be obtained.
In this construction, in connection with the irradiating direction of the image light, each hologram element is bonded in such a fashion that the direction of the light-transmissible film giving a large thermal shrinkage ratio is aligned with the axial direction.
According to the ninth aspect of the present invention, it is preferred that at least one chromatic component of the RGB (red, green and blue) components of the image light irradiated from the illumination device is linearly polarized light.
When two linearly polarized lights, the polarization directions of which cross each other, are incident into the light-transmissible film as the anisotropic material, the component of light is completely divided into two components and travels inside the light-transmissible film.
When two or more components in RGB are linearly polarized rays of light the polarization directions of which cross each other, the image is superposed with a rainbow colored interference pattern because the refractive index of light is different depending on the wavelength.
When at least one chromatic component is linearly polarized light as stipulated in the ninth aspect described above, the present invention operates most effectively.
Next, according to the tenth aspect of the present invention, the hologram element preferably has a diagonal length of 104 cm or more.
Incidentally, degradation of image quality resulting from the interference pattern becomes more severe as the size of the hologram element becomes greater.
In a large-scale screen in which the diagonal length of the hologram element exceeds 104 cm, even a slight degradation of the image is magnified and becomes more noticeable. Therefore, the present invention operates effectively.
When the interference patterns occur in a large-scale screen exceeding 152 cm, in particular, image quality drops extremely and the construction of the present invention operates effectively.
Incidentally, the term xe2x80x9cdiagonal lengthxe2x80x9d represents the length of the diagonal when the hologram screen is rectangular or square.
When the hologram screen has another shape, the present invention exhibits the similar effect when the area of the hologram element coincides with that of a screen having the size described above.
According to the eleventh aspect of the present invention, the hologram element is preferably of a transmission type or a reflection type.
The hologram element is produced from a photosensitive material by use of reference light and object light generated through a light diffusion member. The transmission type element can be produced when object light and reference light are caused to be incident from the same direction. The reflection type element is opposite to the former.
The present invention can be applied to both a transmission type and a reflection type.
According to the twelfth aspect of the present invention, the angle of incidence of the image light to the center of the hologram screen is preferably from 20 to 50 degrees.
In consequence, the image on the hologram screen becomes easier to watch.
When the angle of incidence is less than 20 degrees, the image light is divided into two parts inside the light-transmissible film due to birefringence when it is incident into the hologram screen, and these two parts are more likely to interfere on the hologram element with the result that the interference pattern is more likely to occur.
When the image light is incident at an angle of 50 degrees or more, distortion occurs at the end portions of the image displayed and trapezoidal distortion becomes so strong that the correct image cannot be displayed.
The most preferred angle of incidence is 35 degrees.
The angle of incidence is defined in the following way with reference to the later-appearing FIG. 1. A segment connecting the center G1 of the hologram screen to the irradiation center G2 of the illumination device is referred to as a segment A. A perpendicular at the center G1 of the hologram screen is referred to as a perpendicular B. An arc extending from the segment A to the segment B and having a center at G1 is referred to as an arc C. In this case, the angle of incidence corresponds to the center angle of this arc C.
Next, according to the thirteenth aspect of the present invention, a visible ray transmission factor of the light-transmissible film described above is preferably at least 10%.
In this way, it is possible to exploit the feature of the hologram screen that the image and the background can be simultaneously observed.
When the visible ray transmission factor is less than 10%, the feature described above is likely to be spoiled.
Incidentally, the visible ray transmission factor is determined by attenuation of the intensity of light before and after the incidence of light when the visible rays of light, that is, the rays of light having wavelength of 380 nm to 780 nm, are allowed to be incident into the light-transmissible film.